DISTINCT NFAT FAMILY PROTEINS ARE INVOLVED IN THE NUCLEAR NFAT-DNA BINDING COMPLEXES FROM HUMAN THYMOCYTE SUBSETS

The nuclear factor of activated T cells (NFAT) is involved in the tran scriptional induction of cytokine and other immunoregulatory genes dur ing an immune response, Among four distinct NFAT family members identi fied to date, mRNAs of NFAT1, NFATc, and NFATx are expressed in the th ymus, Here, we report the distribution of these three NFAT family memb ers in human fetal thymocyte subsets and in peripheral mature T cells, We show that NFATx mRNA was expressed in all T lymphocyte subsets tes ted and was highest in CD4(+)CD8(+) double positive (DP) thymocytes, C onversely, NFAT1 mRNA was preferentially expressed in the mature CD4() single positive (SP) populations. NFATc mRNA was present at low leve ls in all subsets but strongly induced upon treatment with phorbol est er and calcium ionophore, Interestingly, we detected NFAT-DNA binding complexes in DP thymocytes, albeit at lower levels than in CD1 SP cell s, Corresponding to the mRNA expression, we observed that NFATx was re sponsible for the NFAT-DNA binding in DP thymocytes. Moreover, this DN A binding was inhibited by cyclosporin A, indicating that NFATx nuclea r translocation was regulated by the calcineurin phosphatase in DP thy mocytes. For the CD1 SP populations, NFAT1 and NFATc, and to some exte nt NFATx, were responsible for the NFAT-DNA binding complexes, These r esults indicate that NFAT family members are differentially regulated during the development of T cells, and that NFATx may play a distinct role in calcineurin dependent signaling in DP thymocytes.